1. Field of the Invention
The invention relates generally to systems and methods for mapping geometrical features. In particular, embodiments of the present invention relate to systems and methods for inspecting screens used in oil and gas industry.
2. Background Art
In the drilling of oil wells, referring to FIG. 1, typically a wellbore 12 is drilled throw a geological formation 14 containing fluids such as oil. The wellbore 12 is protected with a casing 16 having perforations 18 through which fluids flow from formation 14 into the wellbore 12. The fluids are pumped out of the wellbore using any artificial lift, such as gas lift or a pumping system 10, which may be an electro-submersible pump, progressive cavity pump, rod pump, hydraulic jet pump, or diaphragm pump. The fluids flowing into the wellbore often include undesirable particles such as sand, which may result in inefficient oil production due to obstruction of fluid flow, wearing of the pumping system, and casing failure caused by formation corrosion. Therefore, sand control is often necessary.
In order to control sand, usually a sand screen is placed in the wellbore to screen out the sand particles while allowing the fluids to flow through. As shown in FIG. 1, a packer 13 may be used to isolate the pumping system 10 from the completion. The packer 13 has a sand screen 15 attached thereto. Under the action of drawdown produced by the pumping system 10, fluids are forced through the sand screen 13. One of the most commonly used screens is a wire-wrapped screen, formed by helically wrapping wires around a length of perforated base pipe. Sand screens may be attached to a packer that protects a pumping system (as shown in FIG. 1) or to a production string deployed in the production zone. Wire wrapped screens may be used, alone or in combination with gravel-packing, either in open-hole or in cased-hole. A method of constructing wire-wrap well screens has been described in U.S. Patent Application Pub. No. 2005/0125980.
The filtering performance is a direct function of the size of the gap between adjacent turns of wires as only particles smaller than the gap will be allowed to commingle with the fluid produced in the well. The gap also determines fluid flow efficiency because if the gap is too small, fluid flow may be restricted, and the screen may be clogged easily. Referring to FIG. 2A for a typical screen in the art, the gaps 21 between the long wires 22 are generally on the order of 100 microns, and the wire itself usually has a width on the order of 1 millimeter. FIG. 2B illustrates a cross-sectional view of a screen. Wire-wrapped well screens may be of large size, and are often several tens of feet long. Thus, there are usually a large number (tens of thousands) of repetitive patterns within one screen.
Because the widths of gaps 21 between wires 22 are critical for sand control, it is important to inspect the screen in a fast and economical way and subsequently to maintain the screen. The gaps should be as uniform as possible throughout the screen. Conventional inspection techniques involve manual methods using a gauge or an optical scope to measure the gap widths. Due to the small sizes of the gap widths, usually an enlarged image from a magnifying lens is required in order to measure the widths. Some of these techniques have been disclosed, for example, in U.S. Pat. No. 4,914,514. Conventional techniques are often inefficient and are subject to human errors. More recently, automated optical inspection methods have been disclosed, for example, in U.S. Patent Application Pub. No. 2004/0258293, which describes using a camera to capture images of a portion of the screen to determine the distances between wires. Due to the large size of a screen, still only a subset of sample sections may be examined, and statistical techniques are often used to characterize the quality of a screen using quantities such as the average gap width and the statistical variance about the average.
Therefore, there still exists a need for new approaches for inspecting a screen or other large geometrical features in a fast, accurate, and economic way.